1. Field of the Invention
The present invention relates to a filter system which is coupled to a crystallizer in a liquid crystal separation unit and a method for using same for extracting liquid from a liquid crystal slurry to enhance solid crystal recovery. More specifically, the present invention relates to a filter system comprising porous metal tubes which have a very small porosity and which are utilized in a closed feedback loop of liquid-crystal slurry for extracting liquid filtrate from the slurry and returning the higher crystal concentration liquid-crystal slurry back to a crystallizer in a process for the extraction of para-xylene crystals from a mother liquor feed including mixed xylenes and impurities in liquid and crystal form.
2. Description of the Prior Art
Heretofore the extraction of high purity para-xylene crystals from a feed of mixed xylenes and impurities has included the steps of cooling a feed of mixed xylenes and impurities in at least one crystallizer in a crystallization stage to crystallize out para-xylene from the liquid crystal slurry, separating the liquid component comprising ortho-xylene and meta-xylene and impurities from the solid crystal para-xylene in a centrifuge to obtain high purity para-xylene, supplying the mixed liquid (xylenes and impurities) filtrate, including melted para-xylene due to centrifuge work input and heat from the environment, to a holding drum, supplying the all liquid filtrate to an isomerization stage where the filtrate is reacted over a catalyst bed, separating para-xylene and mixed xylenes from impurities in a distillation stage and returning the mixed xylenes to the crystallization stage.
It has been found that the liquid filtrate from the centrifuge contains more para-xylene, e.g., 1.1% in one separation unit, than the solubility of para-xylene at the crystallizing temperature (-85.degree. to -95.degree. F.) indicates should be there. As a result about 0.7 %, for the one separation unit, solid para-xylene crystal recovery is lost in the centrifuge -typically several centrifuges due to their inefficiencies and high feed rates.
As will be described in greater detail hereinafter, the method and apparatus of the present invention provide a filter system through which a portion of a liquid-crystal slurry in a crystallizer is diverted to obtain substantially crystal-free liquid filtrate which is supplied to an isomerization stage and a higher crystal concentration liquid-crystal slurry which is returned to the crystallizer.
Although not known with absolute certainty, it is believed that controlling the solids concentration in the liquid-crystal slurry in the crystallizer using the filter system of the present invention facilitates increased solid crystal para-xylene recovery from a centrifuge of the apparatus.
Preferably, and as described in greater detail hereinafter, the filter system includes porous metal tubes having pores therein with a nominal 10 micron diameter or smaller.
The liquid filtrate from the filter system is supplied to an isomerization stage while liquid filtrate from the centrifuge is returned in a controlled manner to the crystallizer. Periodically, some of the liquid filtrate from the filter system is backflushed through the filter system to unclog the pores which become clogged with very small crystal particles which are then returned to the liquid-crystal slurry that is returned to the crystallizer.